System/Library/PrivateFrameworks/SecurityFoundation.framework

_SFSymmetricKeySpecifier.h

NSObject
>
_SFKeySpecifier

@interface _SFSymmetricKeySpecifier : _SFKeySpecifier {
    id _symmetricKeySpecifierInternal;
}

@property (readonly, nonatomic) long long keyLengthInBytes;

- (void).cxx_destruct;

@end